1. Field of the Invention
The present invention relates to a method of forming solder bumps for packaging a semiconductor device on a mounting substrate, and more particularly to a method of forming solder bumps improved so that a semiconductor device may be packaged on a mounting substrate with high reliability.
2. Description of Related Art
In order to increase packaging density of a semiconductor device thereby to further miniaturize an electronic equipment, flip chip bonding in which a semiconductor device is mounted directly on a mounting substrate such as a printed wiring board and electrodes of the semiconductor device and the printed wiring board are bonded together is being conducted widely.
A solder bump method is one of the methods of flip chip bonding, in which, as shown in FIG. 6, a semiconductor device and a printed wiring board are connected electrically with each other by forming solder ball bumps 22 on Al electrode pads 12 and joining solder ball bumps 22 to electrodes (not shown) of a printed wiring board.
Further, a barrier metal 29 lies between an Al electrode pad 12 of the semiconductor device and the ball bump 22 for the purpose of improvement of adhesion and preventing mutual diffusion. Because of the reason that this barrier metal affects a finished configuration of the solder ball bump, it is called a Ball Limiting Metal (BLM) film. The solder ball bumps form a solder film on the BLM film after the BLM film is formed. The solder film is further treated with heat, and is formed into a predetermined configuration on the BLM film by surface tension of the molten solder.
An example of a method of forming a solder bump will be explained hereinafter with reference to FIG. 7. FIGS. 7A to 7E are sectional views of the substrate in respective processes in executing the solder ball bump method, respectively.
In order to form a solder bump, first, as shown in FIG. 7A, an electrode pad 12 composed of an Al alloy or the like is formed on a silicon substrate 10 by a sputtering method, and then a surface protective layer 11 composed of an insulating film such as a polyimide film and a silicon nitride film is coated on the substrate 10. Then, an opening is formed in the surface protective layer 11, thereby to form a connecting hole for exposing the electrode pad 12, and a BLM film 14 composed of a barrier metal layer is formed with patterning on the electrode pad 12 thereafter.
Next, a resist film 18 is formed on the substrate as shown in FIG. 7B, and is applied with patterning further so as to form an opening portion 16 where the BLM film 14 is exposed.
Next, as shown in FIG. 7C, a solder film 20 is formed on the substrate by vapor deposition or the like. In succession, the resist film 18 is removed by resist peeling and cleaning, and the solder film 20 on the resist film 18 is lifted off at the same time. As a result, the solder film 20 remains behind only in the opening portion 16 (FIG. 7B) as shown in FIG. 7D.
Next, the solder film 20 is dissolved by heat treatment, and the solder film 20 located on the BLM film 14 is transformed into ball-shaped solder, thus forming a solder ball bump 22 as shown in FIG. 7E.
Now, high integration and refinement of a semiconductor device are advanced, and a distance (pitch) between adjacent electrode pads is all the more reduced with the above. On the other hand, it is difficult to make the bump diameter smaller than a certain dimension from a viewpoint of security of reliability of junction strength with a mounting substrate such as a printed wiring board for packaging a semiconductor device.
Therefore, as shown in FIG. 8, in order to avoid mutual contact between adjacent bump 23A and bump 23B, it is being tried that an area for forming a solder bump is secured in a wide area different from the electrode pad, an electrode extension portion 27 having a connecting portion 24 for connecting with an electrode pad at one end, a solder bump foundation portion 25 at another end, and a wiring portion 26 for connecting the connecting portion with the solder bump foundation portion is formed of a barrier metal, and then solder bumps are formed at the solder bump foundation portion.
FIGS. 1A to 1D are sectional views of a substrate at a view I--I portion of FIG. 8 in respective processes when the electrode extension portion is formed.
First, a surface protective film 34 composed of polyimide or silicon nitride film or the like is formed on the semiconductor substrate 32 on which an Al electrode pad 30 is formed, next an Al electrode pad 30 is exposed and a long and narrow connecting hole 36 having the same configuration as that of the electrode extension portion 27 shown in FIG. 8 is formed. Then, a photoresist film 40 is formed on the whole surface of the substrate, and furthermore, patterning by exposure and development is performed, and an opening portion 38 communicating with the connecting hole 36 is opened (see FIG. 1A).
Next, the substrate where the opening portion 38 has been formed is set to a high-frequency plasma processing equipment, sputter-etching (reverse sputtering) with plasma is applied to a photoresist film 40, ions are made to collide with the surface of the photoresist film 40 so as to produce transformation by thermal expansion, and, as shown in Fig. 1B, an overhang shape in which a bore at an opening edge 42 of the opening portion 38 is reduced smaller than the bottom portion of the opening portion 38.
Furthermore, barrier metal layers 46 and 48 that are metal multilayer films composed of Cr, Cu and Au are formed on the substrate. As a result, as shown in Fig. 1C, barrier metal layers 46 and 48 are formed on the exposed Al electrode pad 30 and the photoresist film 40, respectively, but no barrier metal layer is formed on a hole wall face 42 of the opening portion 38 because of the transformation in an overhang shape.
Then, when the substrate is soaked in resist peeling liquid and heating oscillation processing is performed, the photoresist film 40 is removed, and the barrier metal layer 48 which has been formed on the photoresist film 40 is also removed by lift-off at the same time. As a result, as shown in Fig. 1D, the electrode extension portion 46 composed of a barrier metal layer connected to the Al electrode pad 30 is formed.
In the above-mentioned conventional method of forming a solder bump, however, it has been difficult to form a pattern of an electrode extension portion in a predetermined shape by the BLM film and to form a solder ball bump in a predetermined shape on the solder bump foundation portion with high yield.
Thus, it has been difficult to improve the reliability of flip chip bonding when a semiconductor device is packaged, and to improve the product yield of the packaged components.
In the light of such circumstances as described above, it is an object of the present invention to present a method of forming a solder bump capable of flip chip bonding on a semiconductor device with high reliability.
The present inventor has found that the above-mentioned problems generated when a solder bump is formed by a conventional method are caused as described hereunder.
The causes are that lift-off of a resist film and a BLM film is incomplete when the BLM film is lifted off together with the resist film, and residues in no small quantities remain on the substrate surface. In a method of forming a solder bump explained with reference to FIG. 7, a simple circular connecting hole is opened on an Al electrode pad. Whereas, when an electrode extension portion is formed as shown in FIG. 8, a shape including a straight line portion and a curved line portion in which a wiring portion in a straight line form is connected to a circular area of a solder bump foundation portion and an electrode pad is presented. This complicated pattern configuration causes poor lift-off of the resist film and the BLM film.
Namely, when the connecting hole is transformed into an overhang form by sputter-etching as the preprocessing of film formation of the BLM, it is difficult to transform the photoresist opening edge corresponding to the electrode extension portion into a moderate overhang shape over the long whole length being different from a case of a simple circle, thus a BLM film is also formed in a pattern sidewall portion where transformation of the opening edge is insufficient. Therefore, the resist peeling liquid can no longer sufficiently penetrate inside, thus making removal of the resist film insufficient. As a result, as shown in Fig. lE, a large quantity of lift-off residues such as a BLM film are generated.
Thus, the present inventor has found as the result of eager study that the residues described above are removable with an adhesive tape, and come to complete the present invention.